Method of preparing polyester filament material

ABSTRACT

Method of preparing a new synthetic surgical filament material by spinning a mixture of a polyester and an additive.

This invention relates to a process for the manufacture of a polyestercomprising filament material suitable for surgical application, as suchor in woven, braided or knitted form, as well as reinforcing beads. Theinvention further relates to a synthetic surgical material that isbiocompatible with a patient and is biodegradable.

The invented process of spinning a polyester material in the presence ofa certain additive can be used in the production of highly fibrillatedsutures, which due to their fibrillation exhibit a good resorption rateand flexibility important for handling the suture and tying the knot.

Regular structurization at the fibre surface, created in the presentspinningprocess, and preserved in the fibre even after hotdrawing,provides to the fibre a high knot strength.

There exist a number of commercial biocompatible and biodegradablesutures based on polyglycolide (Dexon), copolymer of lactide andglycolide (Vicryl), or lactone of hydroxyethyl glycolic acid (PDS).These well known materials are convenient to handle, and have therequired rate of bioresorption. However there still exists a need fornew biocompatible and biodegradable suture materials.

As follows from literature data, polylactide fibers have already beenproduced, but the rate of degradation of these fibers is too low ascompared with that of Vicryl, Dexon or PDS sutures.

While Vicryl, Dexon and PDS sutures respectively disappear after about90, 120 and 180 days from implantation, the polylactide sutures are notresorbed until after about 8 to 17 months from implantation.

Thus although polylactide is a biocompatible, biodegradable andfiberforming polymer, the polylactide sutures have not so far foundpractical application in surgery.

It is an object of the present invention to provide sutures withadequate tensile strength, high dimensional stability and a rate ofhydrolysis comparable to those of Vicryl, Dexon or PDS sutures. It isanother object of the present invention to provide fibers which have aflexibility higher than those produced according to the standardmethods, yet having high tensile strength and modulus. It is a furtherobject of the present invention to provide sutures having a knotstrength higher than that of fibers produced by standard spinningprocedures.

These and other objects are attained by filament material produced bydry- or wet-spinning a spinning mixture comprising a polyester materialand an additive, particularly a polyurethane material. When wet-spinninga coagulant material is needed. Preferred polyester materials are poly(L-lactide), (PLLA), poly (DL-LACTIDE) (PDLLA), and combinationsthereof, having a viscosity-average molecular weight of at least about3×10⁵ and preferably above 5.0×10⁵ Kg/kmol, as calculated according tothe formule: [n]=5.45×10⁻⁴ M_(v) ⁰.73, for a viscosity measured at 25°C. in trichlormethane.

Examples of additives other than polyurethane materials which can beused in preparing the filament material of this invention, areglycolide, lactide, camphor, benzoic acid-2-hydroxyacetate,hexamethylbenzene, 1,2-cyclohexandione and other low-molecular weightorganic compounds which are preferably soluble in trichlormethane and/ordichlormethane and ethanol, and have a melting temperature in the rangeof 40°to 180° C.

Polyurethane is a preferred additive in the spinning mixture, and assuch may be used in the form of a polyester urethane based onhexamethylene diisocyanate, 1,4-butanediol and a copolymer of lacticacid and ethylene glycol, diethylene glycol or tetramethylene glycol;hexamethylene diisocyanate, 2,4,6-tris(dimethylaminomethyl)phenol andcopolymer of lactic acid and diethylene glycol, ethylene glycol, ortetramethylene glycol, a polyester urethane based on hexamethylenediisocyanate, trimethylol propane and a copolymer of polylactic acid andtetramethylene glycol or a polyester urethane based on4,4'-diphenylmethane diisocyanate, 1,4-butanediol and polytetramethyleneadipate. The concentration of the additive in the polylactide materialmay be in the range of 1 to 45% by weight.

Filament material prepared from these kinds of polylactide/additivemixtures may be formed by dry-spinning the polymer/additive mixture froma solution in a good solvent , in particular in dichloromethane and/ortrichloromethane at room temperature through a spinneret. The filamentmaterial obtained in thereafter, according to a particularly preferredembodiment of the process according to the invention, subjected to ahot-drawing operation applying a drawing ratio within a wide range,particularly up to about 25.

The resulting oriented filaments are strong and owing to their regularlystructurized surface form strong knots.

Owing to extensive fibrillation by virtue of using an additive, thefibres are flexible and easy to handle upon suturing or knotting andhydrolyse much faster than the standard polylactide fibers.

The invented fibers, such as the polylactide comprising fibers, exhibitvery little shrinkage when heated at 37° C. in water for 30 hours,namely, about 1 to 5 percent of their initial length. The inventedspinning process avoids degradation of the polymer during extrusion,resulting in fibers of higher tensile strength.

It is recommended that ethylene oxide be applied for sterilization ofthe fibers, for example polylactide comprising fibers, as high-energyradiation may result in crosslinking and chain scission and somedecrease in tensile strength.

After sterilization with ethylene oxide, the sutures in sealed packages,are subjected to a vacuum of 10⁻⁴ Torr at 70° C. for 1 hour. This avoidsabsorption of sterilizing gas in the sutures.

The filaments according to the present invention have a good tensilestrength of at least 0.4GPa, preferably 0.7 GPA. Some have tensilestrengths as high as 0.8 to 1.0 GPa. The invented fibers are resorbed asto 50% after about 150 days, which rate of hydrolysis is comparable tothat of PDS fibers with comparable thickness and strength.

The filaments according to the invention, such as the polylactidecomprising fibers, may be woven, braided, knitted or used asmonofilaments of general surgical application, may be used asreinforcing beads for the construction of biodegradable tracheal orvascular prostheses, especially for by-pass systems. The polymericadditive material, in particular when being PLLA and/or PDLLA, which canbe converted to filaments particularly by dry-spinning, may be presentin a spinning solution, and this in a concentration of 10 to 40% byweight in dichloromethane and/or trichloromethane, as these two solventseasily dissolve the polylactide/additive mixture with the aboveviscosity-average molecular weight of about 3×10⁵ Kg/kmol at roomtemperature. Spinning polylactide/additive fibers from a solution with aconcentration in the range of 10 to 40% by weight provides amonofilament of reasonable tensile strength, which is in additionregularly structurized due to the melt fracture as schematically shownin the accompanying drawing as obtained (drawing ratio λ=0) and afterhot-drawing at drawing ratios λ of 6, 10 and 20, respectively. Evenhot-drawing at high draw ratios does not completely remove the surfacestructure but results in an extension of the pitch of the helixstructure.

The diameter of the resulting fibre will generally be in the range of0.3 to 1×10⁻⁴ m. Preferred monofilaments have a diameter of about 0.4tol×10⁻⁴ m.

Spinnerets having orifice sizes of 0.2 to 1 mm and a length of thecapillary of 10 mm are suitable for spinning the monofilaments. Indry-spinning from dichlormethane or trichlormethane solutions, thesolution is extruded at room temperature at which the solvent evaporatesslowly. A preferred polymer/additive concentration is 15-25, inparticular about 20% by weight.

The filament is extruded at a speed in the range of 0.02 to 2 mm/min.This gives no orientation to the fibre as spun. After spinning thepolylactide fibers are hot-drawn at a temperature in the range of 45°to200° C., preferably at 110°, 170°, 180° or 200° C., which temperaturedepends on the additive concentration in the polymer and the meltingtemperature of the additive.

The draw ratio λ may be up to 25, preferably 14 to 18. Take-up speed maybe in the range of 0.2 to 1 cm.sec⁻¹ with a strain rate in the range of10⁻³ sec.

The hot-drawing of fibers may be carried out in an electric tube-furnacewith a length of 60 cm under a dry, oxygen-free atmosphere. Hot-drawingat temperatures above 120° C. may reduce the molecular weight of thestarting polymer by 1-2 percent.

The filament can be colored by adding an inert material, e.g. CosmeticViolet No. 2, to the solvent before preparation of the spinningsolution.

The invention is illustrated in and by the following examples

II, III and IV, example I being included for comparison purposes:

EXAMPLE I

Filaments with a regularly structurized surface and having a diameter of0.44×10⁻⁴ m, a tensile strength of 1 GPa, a modulus of 12 GPa, astrength of square knot of 0.6 GPa and an elongation at break of 18%were prepared by spinning poly (L-lactide) from a 20 wt % solution intrichloromethane at room temperature.

The poly(L-lactide) had a viscosity-average molecular weight of 6.0×10⁵.The fibre as spun was hot-drawn at 200° C. to a draw ratio of 20.

EXAMPLE II

Filaments with a regularly structurized surface and having a diameter of0.6×10⁻⁴ m, a tensile strength of 0.8 GPa, a modulus of 9 GPa, astrength of a square knot of 0.5 GPa and an elongation at break of 17%were prepared by spinning poly(L-lactide) which contained 10% by weightof camphor, from a 20 wt % solution in trichloromethane at roomtemperature. The poly(L-lactide) had a viscosity-average molecularweight of 6.0×10⁵. The fibre was drawn at 180° C. to a draw ratio of 4.

After hot-drawing the filaments were extracted in ethanol for 4 hours.No additive was present in the fibre after extraction. The filamentsobtained turned out to have a highly fibrillated structure.

EXAMPLE III

Filaments with a regularly structurized surface and having a diameter of0.7×10⁻⁴ m, a tensile strength of 0.65 GPa, a modulus of 8 GPa, astrength of a square knot of 0.45 GPa and an elongation at break of 19%were prepared by spinning poly(L-lactide) containing 5% by weight ofpolyester urethane from an 18 wt % solution in anhydroustrichlormethane. The poly(L-lactide) had a viscosity-average molecularweight of 4.0×10⁵. The fibre was drawn at 150° C. to a draw ratio of 24.The poly(L-lactide)/polyester urethane monofilament so obtained has ahighly fibrillated structure.

EXAMPLE IV

Highly fibrillated filaments having a diameter of 0.6×10⁻⁴ m, a tensilestrength at break of 0.7 GPa were kept in water at 37° C. for 10 to 200days. After about 150 days the filaments were hydrolysed as to 50%. Thisrate of hydrolysis is comparable to that of PDS filaments of similarstrength and thickness.

We claim:
 1. A process for the manufacture for filament involvingdry-spinning a spinning mixture comprising a polyester material and anadditive, wherein the filament is prepared by dry-spinning, at roomtemperature, a solution in which the polyester material ispoly(L-lactide), PLLA, poly (DL-lactide), PDLLA, or combinationsthereof, in a concentration in the range of 5-70% by weight in thepresence of the additive, the additive being a polyurethane which is abiodegradable polyester urethane material.
 2. A process according toclaim 1, wherein the concentration of the polylactide in the solution isin the range of 10-40% by weight.
 3. A process according to claim 1,wherein CHCL₃ , CH₂ C1₂) or both, is used as the solvent material informing the solution.
 4. A process according to claim 2, wherein CHCL₃ ,CH₂ C1₂, or both, is used as the solvent material in forming thesolution.
 5. A process according in claim 1, wherein the additivematerial used is soluble in CHC1₃, CH₂ C1₂ , C₂ H₅ OH, or combinationsthereof, and has a melt temperature in the range of 40°-180° C.
 6. Aprocess according to claim 2, wherein the additive material used issoluble in CHC1₃, CH₂ C1₂ , C₂ H₅ OH, or combinations thereof, and has amelt temperature in the range of 40°-180° C.
 7. A process according toclaim 3, wherein the additive material used is soluble in CHC1₃ , CH₂C1₂ , C₂ H₅ OH, or combinations thereof, and has a melt temperature inthe range of 40°-180° C.
 8. A process according to claim 1, wherein theconcentration of the additive material in the polyester material is inthe range of 1-45% by weight and the polyester material is apolylactide.
 9. A process according to claim 2, wherein theconcentration of the additive material in the polyester material is inthe range of 1-45% by weight and the polyester material is apolylactide.
 10. A process according to claim 3, wherein theconcentration of the additive material in the polyester material is inthe range of 1-45% by weight and the polyester material is apolylactide.
 11. A process according to claim 1, wherein theconcentration of the polylactide in the solution is in the range of 20%by weight.